Surface acoustic wave device

ABSTRACT

A surface acoustic wave device includes a semiconductor substrate, a piezoelectric film disposed on the semiconductor substrate, at least one pair of comb-shaped electrodes constituting an input transducer and an output transducer, a plurality of surface acoustic wave filters having different bands which include a plurality of metal electrodes, so disposed that each of them is located between the input and output comb-shaped electrodes of each of the surface, acoustic wave filters, a bias voltage applying arrangement for applying bias voltages to each of the metal electrodes; and a bias voltage regulating arrangement for regulating the bias voltages.

This is a continuation of application Ser. No. 785,938, filed Oct. 9,1985, now U.S. Pat. No. 4,697,115.

FIELD OF THE INVENTION

This invention relates to a surface acoustic wave (hereinbelowabbreviated to SAW) device which includes a semiconductor substrate, apiezoelectric film disposed thereon and at least a pair of in/outputtransducers disposed at the neighborhood of the extremities of thesurface thereof, and more specifically to a SAW device used as avariable band SAW filter consisting of a plurality of SAW filters havingdifferent bands, a SAW delay line, etc.

BACKGROUND OF THE INVENTION

SAW filters are widely utilized as IF filters in television receiversand filters in various kinds of communication apparatuses. But they areusually as a fixed band filter.

FIG. 7 shows a perspective view indicating the general form of the usualSAW filter, in which the reference numeral 1 is a piezoelectricsubstrate; 2' indicates comb-shaped electrodes constituting an inputtransducer 2; and 3' indicates comb-shaped electrodes constituting anoutput transducer 3.

Since the band of such a filter is determined by the form and number ofpairs of comb-shaped electrodes, it is a constant proper to the SAWfilter and it is not possible to vary it. However, variable band filtersare strongly desired for communication apparatuses, in which a frequencyband varies with time, and for various sorts of multi-channelcommunication apparatuses.

FIG. 8(a) is a top view illustrating a representative variable band SAWfilter, in which corresponding items are indicated by the same referencenumerals as in FIG. 7; 4 represents a switching circuit; 5 representsthe input terminal; and 6 represents the output terminal. In the SAWfilter indicated in FIG. 8(a) a plurality of SAW filters are mounted ona substrate so that the bands of the filters are adjacent to each other,and one of the bands is selected by switching an external circuit 4.FIG. 8(b) shows five curves representing the relation between frequencyand the output, when the contact of the switch in the switching circuitis respectively positioned at A, B, C, D and E in FIG. 8(a).

For such a prior art filter an external circuit 4 for switchinhg isinevitable, what is problematical with respect to the manufacturing costand space saving. In addition, it has as a drawback that the freedom forthe shape of the pass band is small, because it is controlled only byswitching to select a single output transducer.

Further, a SAW delay line is used for signal processing in radar devicesand as a delay line in an SAW oscillator. In particular, development ofa device for which the delay time can be varied is desired, because sucha device can be applied to a variable frequency oscillator and to aghost canceler a television receiver.

FIGS. 20 and 21 illustrate representative prior art devices used asvariable delay lines, in which the reference numeral 1 represents apiezoelectric substrate; 2' indicates comb-shaped electrodesconstituting the input transducer 2; 3A, 3B, . . . are comb-shapedelectrodes constituting output transducers 3; 4 is a switching circuit;5 is the input terminal; and 6 is the output terminal.

FIG. 20 illustrates a so-called delay line with taps, for which the areaof the substrate may be small, but which has a disadvantage that asinterference between different taps is produced due to reflection of aSAW at each of the taps (output comb-shaped electrodes 3A, 3B, . . . ).On the other hand, for the device illustrated in FIG. 21, althoughinterference occurring between the output comb-shaped electrodes 3A, 3B,. . . is small, a disadvantage is that a large piezoelectric substrate 1is necessary. Further, for both the devices an external circuit 4 forswitching is necessary and thus they are problematical with respect tomanufacturing cost and space saving.

OBJECT OF THE INVENTION

An object of this invention is to provide a variable delay time SAWdelay line requiring no external circuit and acting as an amelioratedSAW device which does not have the drawbacks of the prior art techniquesdescribed above.

Another object of this invention is to provide a variable band SAWfilter requiring no switching circuit such as that indicated in FIG. 8and having a large freedom.

SUMMARY OF THE INVENTION

In order to achieve this object of this invention, a SAW deviceaccording to this invention comprises metal electrodes between the inputand output transducers on a piezoelectric film, an arrangement forapplying bias voltages between each of the metal electrodes and thesemiconductor substrate, and an arrangement for controlling the biasvoltages.

The part where the metal electrodes are disposed has a structureso-called monolithic MIS (Metal/Insulator/Semiconductor). Thepropagation loss of a SAW propagating in such a structure variesconsiderably, depending on the bias voltages applied between the metalelectrodes and the semiconductor substrate. FIG. 9 shows an example ofrelations between the propagation loss and the bias voltage with aparameter of temperature. As indicated in this figure, the propagationloss increases rapidly in a certain voltage domain. The domain where theSAW is attenuated rapidly corresponds to a voltage domain where thesurface of the semiconductor (interface piezoelectricsubstance/semiconductor body) is strongly inverted. FIG. 10 illustratesa comparison of a C-V characteristic (capacity-voltage characteristic)curve (curve b) with the propagation loss (curve a). As can be seen inthe figure, the propagation loss increases rapidly, when thesemiconductor surface is strongly inverted (domain at the left side ofthe broken line).

Therefore, by applying a great bias voltage producing a stronglyinverted domain a metal electrode, it is possible to cut off a SAW inthis domain. Further, since it acts as a variable attenuator dependingon the bias voltage, when the applied bias voltage is not so great, itis possible to obtain band characteristics having a large freedom and tovary continuously the propagation loss of a SAW by regulating biasvoltages applied to each of the metal electrodes.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a perspective view of a variable band SAW filter according toan embodiment of this invention;

FIG. 2 is a diagram showing a relation between the output and frequencyfor the filter of FIG. 1;

FIG. 3 is a top view of a SAW filter according to another embodiment ofthis invention;

FIGS. 4, 5 and 6 are partial top views of SAW filters according to threeother different embodiments of this invention;

FIG. 7 is a perspective view of a fixed band prior art SAW filter;

FIG. 8A is a top view of a variable band prior art SAW filter and

FIG. 8B is a diagram illustrating the relation between the output andfrequency for the filter of FIG. 8A;

FIG. 9 shows graphs illustrating the relation between the propagationloss and the bias voltage with the parameter of temperature;

FIG. 10 shows graphs illustrating the relation between the bias voltageand the propagation loss as well as the high frequency capacitance;

FIG. 11 is a perspective view illustrating a variable SAW delay lineaccording to an additional embodiment of this invention;

FIG. 12 is a top view illustrating a variable SAW delay line accordingto another embodiment of this invention;

FIGS. 13 to 16 are partial top views of variable SAW delay linesaccording to four other different embodiments of this invention;

FIGS. 17 to 19 are cross-sectional views of variable SAW delay linesaccording to three other different embodiments of this invention; and

FIGS. 20 and 21 are top views illustrating two prior art variable SAWdelay lines.

DETAILED DESCRIPTION

Hereinbelow, this invention will be described more in detail in thelight of some embodiments, referring to the drawings, but they are notat all intended to be restrictive and it should be understood thatvarious modifications and ameliorations are possible without departingfrom the scope of this invention.

FIG. 1 is a perspective view illustrating an embodiment of a variableband SAW filter according to this invention. In the figure,corresponding items are designated by the same reference numerals asused in FIG. 8. A semiconductor substrate 7 and a piezoelectric film 8disposed thereon constitute a piezoelectric substrate 1. On the othersurface, which is opposite to the piezoelectric film 8, of thesemiconductor substrate 7 is disposed a back side electrode 9, which isgrounded. At one end of the surface of the piezoelectric substrate 1 aredisposed comb-shaped electrodes 2' constituting the input transducer 2,and at the other are comb-shaped electrodes 3A, 3B, 3C, 3D constitutingthe output transducer 3, which are so formed that each of them respondsonly to a SAW within a respective one of four bands which differ fromeach other. All the output comb-shaped electrodes 3A, 3B, 3C, 3D areconnected to the output terminal 6. Between the input comb-shapedelectrodes 2' and the output comb-shaped electrodes 3A, 3B, 3C, 3D aredisposed metal electrodes 10A, 10B, 10C, 10D. A surface acoustic wavetravelling from the input transducer 2 to the output transducer 3 willhave portions which travel along respective paths of travel 15A, 15B,15C, 15D to the respective electrodes 3A, 3B, 3C, 3D, and each of theelectrodes 10A, 10B, 10C, 10D is elongate and is positioned over arespective path of travel 15A, 15B, 15C, 15D so as to extend parallelthereto. For each of the filters one of four bias voltages V_(A), V_(B),V_(C), V_(D) is applied between a respective one of the metal electrodesand the back side electrode 9. The band of the whole filter is varied bycontrolling each bias voltage. When the bias voltages V_(A), V_(B),V_(C), V_(D) applied to the metal electrodes 10A, 10B, 10C, 10D arevaried, the propagation loss due to the effect of each of the filtersvaries, and therefore the whole pass band can be set differently byadjusting the bias voltages.

FIG. 2 shows variations of the propagation loss, due to the effect ofthe filters in the bands A, B, C, D for different bias voltages. Thebias voltages canbe set selectively so that a desired output can beobtained, e.g. as indicated by the solid line.

In the device indicated in FIG. 1 the output transducer 3 is dividedinto a plurality of pairs of comb-shaped electrodes so that the wholeoutput power is taken out as the sum of the outputs in parallel, butalso a structure such as is indicated in FIG. 3 is possible.

On the back side surface of the device indicated in FIG. 3 is disposed aback side electrode, which is grounded, just as described in thepreceding embodiment, and the bias voltages V_(A), V_(B), V_(C), V_(D)and V_(E) are applied to the metal electrodes 10A, 10B, 10C, 10D and10E, respectively, in the same way as they are for the precedingembodiment. The difference between the device indicated in FIG. 3 andthat indicated in FIG. 1 consists only in that the output transducer 3consists of only one pair of comb-shaped electrodes 3' and that thepitch or spacing of the comb-shaped electrodes 3' varies discontinuouslyin a direction perpendicular to the propagation direction a SAW. Each ofthe pitches of the electrodes corresponds to one of the bands A, B, C,D, E and thus the device indicated in FIG. 3 works in the same way asthat indicated in FIG. 1. The advantage of this form is that the bandscan be divided more finely and that bonding steps for taking out theoutput to the exterior can be reduced.

Furthermore, if the metal electrodes 10 are formed to be finer and aredisposed so as to be closely adjacent to each other and if the pitch ofthe comb-shaped electrodes 3' varies continuously in the directionperpendicular to the propagation direction of a SAW, it is possible toadjust selectively the pass band by selecting the distribution of thebias voltages. The semiconductor substrate in the devices indicated inFIGS. 1, 3 and 4 can include an insulating film such as an oxide film,nitride film, etc. obtained by oxidizing or nitriding its surface.

Further, the material of the metal electrodes can be the same as that ofthe comb-shaped electrodes and by the fabrication they are produced bythe same process a photolithographic process) and at the same time asthe comb-shaped electrodes.

Moreover, although the input transducer indicated in FIGS. 1, 3 and 4consists of only one pair of comb-shaped electrodes, which are common toall the filters, it can consist of a plurality of pairs of comb-shapedelectrodes 2A, 2B, 2C, 2D, as indicated in FIG. 5, having the same inputcharacteristics.

Further, the input transducer can consists of a plurality of pairs ofcomb-shaped electrodes 2A, 2B, 2C, 2D having different inputcharacteristics. However, in this case, each pair of comb-shapedelectrodes must be matched by using separate matching circuits 11A, 11B,11C, 11D.

In addition, it is desirable to form the end surface of each of themetal electrodes 10A, 10B, . . . so that it is inclined with respect tothe propagation direction of a SAW, because influences due to thereflection of a SAW at the end surface is reduced in this way.

FIG. 11 is a perspective view of variable SAW delay lines according tothis invention.

A semiconductor 7 and a piezoelectric film 8 disposed thereon constitutea piezoelectric substrate 1. On the other surface, which is opposite tothe piezoelectric film 8, of the semiconductor substrate 7 is disposed aback side electrode 9, which is grounded. At one end of the surface ofthe piezoelectric substrate 1 are disposed comb-shaped electrodes 2'constituting the input transducer 2 and at the other end are comb-shapedelectrodes 3A, 3B, 3C, 3D constituting the output transducer 3. Theoutput distances between the electrodes 3A, 3B, 3C, 3D and therespective input comb-shaped electrodes 2 are different from each otherso that the times necessary for a SAW emitted by the transducer 2 toreach the output transducers 3A, 3B, 3C, 3D are different from eachother. All the electrodes 3A, 3B, 3C, 3D are connected to the outputterminal 6. Metal electrodes 10A, 10B, 10C and 10D are mounted on thepropagation paths of surface acoustic waves traveling from the inputtransducer 2 to the electrodes 3A, 3B, 3C and 3D, respectively, and biasvoltages V_(A), V_(B), V_(C) and V_(D) are applied to these metalelectrodes, respectively. According to the working principle describedabove, when the bias voltages V_(A), V_(B), V_(C), V_(D) are so set thatthe surface of the semiconductor substrate is strongly inverted, surfaceacoustic waves propagating in the portions under the metal electrodes10A, 10B, 10C, 10D are rapidly attenuated. To the contrary, by settingthe bias voltages so that the surface of the semiconductor substrate isat the depletion state or weakly inverted or at a sufficiently chargedstate, it is also possible to make the attenuation of a SAW sufficientlysmall. Consequently, by controlling the bias voltages V_(A), V_(B),V_(C), V_(D) applied to the metal electrodes 10A, 10B, 10C, 10D, it ispossible to selectively permit and prevent surface acoustic waves fromreaching the output comb-shaped electrodes 3A, 3B, 3C, 3D. Further,since it is also possible to vary continuously the propagation loss ofSAW by regulating the bias voltages V_(A), V_(B), V_(C), V_(D), theoutput level of each of the delay lines can be suitably controlled.

In this case, since it is possible to control the propagation of SAWonly by commuting the bias voltages, the device according to thisinvention has an advantage that the circuit therefor can be simplifiedwith respect to that of the prior art device, by which switching of SAWis effected by switching of RF signals. Further, since the semiconductorsubstrate is cheaper and large size substrates are available more easilywith respect to the piezoelectric body, the substrate according to thisinvention has as an advantage that it can be fabricated with a lowercost than the substrate used by the prior art techniques.

Although in the device illustrated in FIG. 11, the output transducer 3is divided into a plurality of pairs of comb-shaped electrodes 3A, 3B,3C, 3D and the final output is taken out in the form of a resultantoutput of plural circuits connected in parallel, the structure asillustrated in FIG. 12 is also possible.

In the device illustrated in FIG. 12 the output transducer 3 consists ofone pair of comb-shaped electrodes 3', which are constructed in the formof stairs so that the distance between the input transducer and theoutput comb-shaped electrodes in the propagation direction of a SAWvaries discontinuously in the direction perpendicular to the propagationdirection of a SAW. It is clear that the device illustrated in FIG. 12works in completely the same manner as that illustrated in FIG. 11.

Furthermore, the output comb-shaped electrodes 3' can be formed asillustrated in FIG. 13.

In the devices illustrated in FIGS. 12 and 13 the comb-shaped electrodes3' of the input transducer 3 are so disposed that the distances betweenthe comb-shaped electrodes 2' of the input transducer 2 and thecomb-shaped electrodes 3' of the output transducer 3 respectively varydiscontinuously or continuously in the direction perpendicular to thepropagation direction of SAW. In this way, it is possible to obtain asadvantages that the delay time can be regulated more accurately and thatbonding steps for taking out the output to the exterior can be reduced.

Similarly the input transducer 2 may be divided into a plurality ofelectrodes 2A, 2B, 2C, 2D, as indicated in FIG. 14, or only one pair ofelectrodes 2' may be constructed in the form of stairs, as indicated inFIG. 15, or they may be inclined with respect to the propagation path ofa SAW, as indicated in FIG. 16.

Furthermore, it is desirable that the end surfaces of the metalelectrodes 10A, 10B, . . . be inclined with respect to the propagationdirection of a SAW, because in this way influences due to the reflectionof a SAW at the end surfaces are reduced.

The semiconductor substrate 7 used for this invention can be covered byan insulating film 11, such as an oxide film, a nitride film, etc. byoxidizing or nitriding its surface, as indicated in FIG. 17.

In addition, for the piezoelectric substrate 1, as illustrated in FIG.18, the propagation path of a SAW can be divided into two regions, inone of which metal electrodes 10A, 10B, . . . are disposed forcontrolling the propagation of the SAW and in the other of which a metalfilm 12 is disposed at the interface between the piezoelectric film 8and the semiconductor substrate 7 or the insulating film 11. For such astructure, in the portion where a metal film 12 exists, the potential ofa SAW is shielded and thus the semiconductor substrate and SAW do notinterfere with each other. This structure is advantageous, when a longdelay time is required, because attenuation of a SAW is extremely small.

Furthermore, as indicated in FIG. 19, it is also possible to use anothersubstrate structure, by which the piezoelectric film is disposed on theportion where the input comb-shaped electrodes 2' and the metalelectrodes 10A, 10B, . . . exist but is not on the propagation paths ofsurface acoustic words therebetween. This structure is advantageous,similarly to that indicated in FIG. 19, when a long delay time isrequired, because a SAW propagates through the crystal semiconductorsubstrate 7, where the piezoelectric film 8 does not exist, and thus theattenuation of the SAW is extremely small there.

As explained above, according to this invention, space saving and costreduction can be achieved with respect to prior art variable band SAWfilters, because it is not necessary to provide separate switchingelements for each of the filters for selecting arbitrarily an output. Inthe device according to this invention, since a function equivalent tothe switching can be realized only by switching on/off the biasvoltages, the circuit itself is simplified. Further, since thepropagation loss for each of the bands can infinitely varied bycontrolling the bias voltages in the analog manner, band characteristicsof large freedom in selection can be obtained.

SAW filters according to this invention can be applied to all sorts ofapparatuses where SAW filters are used. However, they are usefulspecifically for communication apparatuses, in which the frequency bandis varied in time, such as CATV, communication satellite, relay,tranceiver, and multi-channel communication apparatuses.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. A surface acoustic wave device comprising:a semiconductor substrate; A piezoelectric film provided on said semiconductor substrate; an input transducer and an output transducer provided at spaced locations on said piezoelectric film, wherein a surface acoustic wave travelling through said piezoelectric film from said input transducer to said output transducer has respective portions which travel along respective paths of travel; a plurality of elongate metal electrodes provided on said piezoelectric film between said input transducer and said output transducer, each said metal electrode being positioned along a respective said surface acoustic wave path of travel so as to extend parallel to a direction in which surface acoustic waves travel along the path; bias voltage applying means for applying to each of said metal electrodes a respective bias voltage; and bias voltage regulating means for independently regulating said bias voltages so as to control the respective propagation losses of surface acoustic waves traveling past each said metal electrode along a respective said path of travel.
 2. A surface acoustic wave device according to claim 1, wherein said input transducer includes at least one pair of comb-shaped input electrodes, and wherein said output transducer includes a plurality of pairs of comb-shaped output electrodes which are disposed so that each corresponds to a respective one of said metal electrodes and receives surface acoustic waves propagating along a respective one of said paths of travel.
 3. A surface acoustic wave device according to claim 1, wherein said input transducer and said output transducer each include a single pair of comb-shaped electrodes having a plurality of fingers, said comb-shaped electrodes of said output transducer being formed so that the spacing between the fingers varies discontinuously in a direction approximately perpendicular to said paths of travel of surface acoustic waves.
 4. A surface acoustic wave device according to claim 1, wherein said input transducer and said output transducer each include a single pair of comb-shaped electrodes having a plurality of fingers, said comb-shaped electrodes of said output transducer being so formed that the spacing between the fingers varies continuously in a direction approximately perpendicular to said paths of travel of surface acoustic waves.
 5. A surface acoustic wave device according to claim 1, wherein said input transducer includes a plurality of pairs of comb-shaped input electrodes which have identical input characteristics and which are disposed so that each corresponds to a respective one of said metal electrodes and produces surface acoustic waves which travel along a respective one of said paths of travel.
 6. A surface acoustic wave device according to claim 1, wherein said input transducer includes a plurality of pairs of comb-shaped input electrodes which have different input characteristics and which are disposed so that each corresponds to a respective one of said metal electrodes and produces surface acoustic waves which travel along a respective one of said paths of travel, each said pair of said comb-shaped input electrodes being provided with an input signal through a respective matching circuit.
 7. A surface acoustic wave device according to claim 1, wherein said input transducer includes at least one pair of comb-shaped input electrodes, wherein said output transducer includes a plurality of pairs of comb-shaped output electrodes which are disposed so that each corresponds to a respective one of said metal electrodes and is responsive to surface acoustic waves which travel along a respective one of said paths of travel, and wherein said paths of travel of surface acoustic waves between said input electrode and said output electrodes are different from each other.
 8. A surface acoustic wave device according to claim 1, wherein said input transducer includes at least one pair of comb-shaped input electrodes and said output transducer includes at least one pair of comb-shaped output electrodes, said comb-shaped output electrodes being positioned so that the distance between each said pair of input electrodes and a corresponding said pair of output electrodes varies in a direction perpendicular to said paths of travel of surface acoustic waves.
 9. A surface acoustic wave device according to claim 1, including an insulating film provided between said semiconductor substrate and said piezoelectric film.
 10. A surface acoustic wave device according to claim 1, wherein said semiconductor substrate has first and second surfaces provided on opposite sides thereof, said piezoelectric film being provided on said first surface of said semiconductor substrate, and wherein said bias voltage applying means includes an earth electrode provided on said second surface of said semiconductor substrate, said bias voltage applying means applying each said bias voltage between said earth electrode and a respective one of said metal electrodes. 